A. Field of the Invention
The present invention relates generally to magnetically activated latches and more specifically to molded-case circuit breaker devices having current-limiting blow-off characteristics equipped with anti-rebound mechanisms.
B. Description of the Prior Art
In the field of electrical circuit breaker technology, it is frequently necessary to provide a circuit breaker that is responsive to a specific type of current overload abnormality. There are three basic types of current interruption mechanisms--thermal, electromagnetic, and blow-off-type mechanisms. For example, a circuit breaker may be designed to automatically trip open after a relatively long period of moderate current overload through the breaker. Such a circuit breaker typically will have a bimetallic thermal element in it which gradually heats due to the moderate overload current to bend the bimetallic element and eventually cause a circuit breaker to trip open. Such circuit breakers are useful in handling moderate current overloads of 100% to 500% of the rated load of the breaker.
Other current interruption mechanisms known as instantaneous trip breakers are used to handle situations in which the current overload is on the order of 500% to 600% of the rated current of the breaker. This type of circuit breaker is typically a magnetic mechanism which reacts in a matter of milliseconds to excessive current overloads. Such a circuit breaker may either automatically reset or require manual reclosing after the excessive current has subsided.
Yet another type of current interrupting mechanism commonly called a blow-off mechanism is commonly used to handle massive overcurrent conditions and instantaneously open during the first milliseconds that a massive overcurrent condition exists. An example of a current-limiting, blow-off-type mechanism is described in U.S. Pat. No. 4,071,836 to Cook et al.
In the event of a massive overcurrent condition, it is imperative to instantaneously open the circuit breaker contacts because any delay in opening allows excessive currents to flow through the electrical contacts of the circuit breaker which may cause the contacts to separate and then reclose and fuse together, thereby permanently preventing the contacts from being opened. Arcing between circuit breaker contacts during opening may further cause instantaneous heating of the electrical contacts so that it is undesirable to allow the circuit breaker to reclose immediately after the massive overcurrent has subsided because the contacts may still be hot enough for unwanted fusing to occur. Thus, it is desirable to provide an anti-rebound mechanism to prevent automatic reclosing of blow-off-type mechanisms. An example of a typical anti-rebound mechanism is described in U.S. Pat. No. 4,144,513 to Shaffer et al. The present device is similar to typical prior art anti-rebound mechanisms in that both involve the use of an anti-rebound latch which is somehow biased toward a contact arm to move the latch into a notch in the contact arm when a contact arm of the circuit breaker is blown off by electrodynamic forces. The prior art, however, as exemplified by Shaffer et al. uses a biasing spring to exert a constant pressure on the latch at all times. In the particular device described in the Shaffer patent, as the contact arm is blown open, a camming surface at one end of the contact arm engages an anti-rebound latch pin forcing it against the force of a biasing spring. Once an upper end of the camming surface moves below the pin, the force of the biasing spring moves the pin into a notch. Cooperation between the pin and the notch locks the contact arm against further movement. The force of the biasing spring in the device described by the Shaffer patent creates unwanted friction between the latch and the contact arm resulting in undesirable wear between the latch and the current-carrying arm during normal operation of the circuit breaker in the absence of excessive current surges. For years, anti-rebound mechanisms have relied on a spring-type biasing which produces unnecessarily constant forces to engage the anti-rebound mechanism in the event of a massive overcurrent.